Energy harvesters have long been used to extract energy from the local environment, as in the case of windmills and water turbines, and convert it to mechanical or electrical power. Modern micro-energy harvesters convert heat, sunlight, radio frequency energy, or vibration into electricity via thermoelectric generators (TEGs), photovoltaic panels, radio frequency (RF) harvesters, or piezoelectric generators, respectively. Power levels ranging from microwatts to hundreds of milliwatts are harvested by the generator within the micro-energy harvester and then converted by a DC-DC voltage converter into a load voltage. Since the arrival of digital integrated circuits, electronic products that can operate from decreasing amounts of energy have proliferated, among them wireless sensors. Wireless sensors have become useful in the fields of personal health, wilderness, and industrial monitoring, and micro-energy harvesters are a natural solution to these new applications. Because energy harvesters can supply power indefinitely, and can be placed in remote or wilderness locations, reliability and longevity have become critical requirements. Energy harvesters may also be used inside buildings where electricity is available, but economic, mobility, and other advantages make energy harvesters a preferred source of electrical power. Unfortunately, peak power is not always available, such as when clouds reduce the irradiance of a solar cell, or when a hot pipe being tapped by a TEG becomes cool. To alleviate environmental variability, storage elements such as capacitors and batteries are often employed to store some of the harvested energy and supplement the harvester during off-peak times. Unfortunately, storage elements may be bulky, expensive, and require periodic maintenance.
Addressing the drawbacks associated with storage elements, more efficient DC-DC converters based on FET switching technologies became available in the 1980s, expanding the applicability of micro-energy harvesters and reducing the need for storage elements. Switching DC-DC converters eliminate heavy transformers and reduce the need for linear voltage regulators, creating a smaller, lighter package and harvesting more power. Inductors and capacitors are used as charge elements, transferring power from the generator to the load through low-loss transistors switched at an appropriate frequency and duty cycle. The size of the charge element(s), duty cycle, and switching frequency determine the input voltage required of the generator for a desired converter output voltage. Once these circuit values are chosen, output power efficiencies of 80-95% may be achieved. Output power efficiency may be defined as the ratio of the output power to the input power. However, efficiencies drop sharply if the generator source voltage varies from the input voltage for which the converter was designed, resulting in lower output power. Additionally, each converter may have a minimum start-up voltage below which the input voltage is insufficient to charge the converter into steady state operation. Unfortunately, a fixed DC-DC converter is limited to delivering high power efficiencies only over a relatively narrow range and must therefore be customized for each generator's source voltage.
Further complicating the challenge of operating a DC-DC converter over a wide range of source voltages, consider the thermoelectric generator (TEG). TEGs extract power from a heat flow caused by a temperature difference, abbreviated as ΔT, established between a heat source and a heat sink. The source voltage is approximately proportional to the ΔT. TEGs commonly operate from ΔTs as little as 5 K to 100 K or more, producing source voltages from millivolts to volts. A TEG source voltage may vary over a 10:1 range or more, depending on the intensity of the heat source, whereas a photovoltaic generator has a relatively stable output voltage—its current varying with solar irradiance. Additionally, as ΔT increases, source voltage increases and source impedance may vary. The variation in source impedance explains another cause of converter inefficiency. One possible solution to maintain maximum power transfer is to change the converter input impedance to match the source impedance. Maximum power transfer occurs when the load impedance of the DC-DC converter equals the source impedance of the generator. Under maximum power transfer conditions, the open circuit source voltage is divided equally between the internal source impedance and the converter's load impedance. In conclusion, conventional DC-DC converters are efficient over a narrow range of input voltages, and a TEG has a particularly wide range of source voltages and, additionally, a shifting source impedance. Thus, what is needed is an improved voltage converter that can accommodate a wide range of input voltages and a shifting source impedance.
In order to align a converter to a generator's source voltage, manufacturers provide designs which allow the customer to choose certain element values that are external to the semiconductor package. For example, referring to FIG. 1, one converter, designated as the LTC3105 contains a boost circuit to step up the voltage, and is generally intended for photovoltaic applications. L1 may be chosen to be between 4.7 μH (micro-henrys) and 30 μH, with a nominal recommended value of 10 μH, depending on the expected source voltage.
For a very low input voltage, a larger L1 value provides higher efficiency and a lower start-up voltage than if the nominal value for L1 is used. The input voltages for which efficiency is >80% ranges from about 0.9 V (volts) to about 2.8 V, or about 3:1, as shown in the graph of FIG. 2. Also, the graph of FIG. 2 indicates that the start-up voltage is about 0.6 V, and then efficiency climbs quickly as input voltage rises, leveling off in the 80-90% range, then dropping off. The choice of external charge element values allows a generator to be nominally matched to a converter. However, the range of generator source voltages over which efficiency is high is still limited, for example, to about 3:1, in the case of the LT'C3105 shown in FIG. 1. For a TEG with a 10:1 range of source voltage, the LTC3105 may have too narrow of an operating range, losing much of the power that could have been harvested.
In order to improve the matching of generator source voltage to a compatible DC-DC converter, having already optimized external component values, some converters provide an adjustable start-up voltage settable by a reference resistor, shown as RMPPC in FIG. 1. In this example, a control circuit called the MPPC (maximum power point control) circuit regulates the average inductor (L1) current within the boost circuit in order to configure the input impedance and start-up voltage of the converter. For example, the start-up voltage can be set to as low as 0.25 V by setting RMPPC to 22 kΩ. Unfortunately, setting the minimum start-up voltage to 0.25 V, in the case of the LTC3105, results in virtually no increase in boost circuit output power as input voltages become several times the minimum start-up voltage resulting in poor efficiency at input voltages greater than the turn on voltage. What is needed is a method of establishing a low start-up voltage to capture the low end of a TEGs operating range, and then extend the operating range to well above the start-up voltage.
An additional problem is the case where the input voltage momentarily exceeds the start-up voltage and then drops below it before the boost circuit has been charged enough to generate the regulated power supplies that power its internal circuitry. If a load, such as a wireless sensor, is connected directly to the boost circuit, it may begin to drain off some of the input energy being used to charge up the boost circuit and thereby sabotage the start-up process, thus delaying the start-up process. Also, if the input voltage drops below the start-up voltage after steady state operation has been established, the load may fully discharge the boost circuit unnecessarily. What is needed is a method of isolating the load from the boost circuit during positive and momentary negative excursions of input voltage occurring across the start-up voltage threshold.
Another solution to environmental variability in harvested power is to combine two or more complementary generators whose off-peak output conditions occur at different times of the day. For instance, a TEG and a photovoltaic cell could be combined to make a more reliable harvester, thus requiring a smaller storage element. In this case, it is desirable for both generators to use the same voltage converter in order to save cost and reduce bulk. However, a photovoltaic cell tends to have a different source voltage than a TEG, thus compounding the problem of DC-DC converters not accommodating a wide enough range of input voltages. However, if one generator could set a boost circuit operating point ideal for its source voltage when it was dominant, and the other generator could set a boost circuit operating point ideal for its source voltage when its dominant, a more compact and reliable energy harvester could be achieved.
One option is to apply microprocessors or digital microcontrollers to the voltage converter in an attempt to optimize its operation through programmed values of operating points, or through switching in and out different components for different operating points. However, the micro-energy harvester is operating in a frugal and small-footprint environment, sometimes operating at far below 100 μW of power, and may require very judicious application of additional power drain for a microprocessor and switching circuitry.
As can be seen, there exists a need in the art for a system and method of dynamically adjusting the set point of a boost circuit according to the instantaneous input voltage such that high output power efficiency may be achieved over a relatively large range of input voltages. Additionally, there exists a need in the art for a system and method of matching the varying source impedance of a TEG to the boost circuit such that maximum power transfer may occur. Furthermore, there exists a need in the art for a system and method of isolating the boost circuit from the load during positive and momentary negative excursions of input voltage around the start-up voltage. Ideally, the system and method require minimal power, are relatively inexpensive, and are easily implemented in a DC-DC converter.